From 5d73448bb7f3d326f310e6f35030821b103b1cdb Mon Sep 17 00:00:00 2001 From: Andrew Trick Date: Fri, 9 Dec 2011 06:19:40 +0000 Subject: Add -unroll-runtime for unrolling loops with run-time trip counts. Patch by Brendon Cahoon! This extends the existing LoopUnroll and LoopUnrollPass. Brendon measured no regressions in the llvm test suite with -unroll-runtime enabled. This implementation works by using the existing loop unrolling code to unroll the loop by a power-of-two (default 8). It generates an if-then-else sequence of code prior to the loop to execute the extra iterations before entering the unrolled loop. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146245 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Transforms/Utils/LoopUnrollRuntime.cpp | 375 +++++++++++++++++++++++++++++ 1 file changed, 375 insertions(+) create mode 100644 lib/Transforms/Utils/LoopUnrollRuntime.cpp (limited to 'lib/Transforms/Utils/LoopUnrollRuntime.cpp') diff --git a/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/lib/Transforms/Utils/LoopUnrollRuntime.cpp new file mode 100644 index 0000000000..2b92e59c3d --- /dev/null +++ b/lib/Transforms/Utils/LoopUnrollRuntime.cpp @@ -0,0 +1,375 @@ +//===-- UnrollLoopRuntime.cpp - Runtime Loop unrolling utilities ----------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file implements some loop unrolling utilities for loops with run-time +// trip counts. See LoopUnroll.cpp for unrolling loops with compile-time +// trip counts. +// +// The functions in this file are used to generate extra code when the +// run-time trip count modulo the unroll factor is not 0. When this is the +// case, we need to generate code to execute these 'left over' iterations. +// +// The current strategy generates an if-then-else sequence prior to the +// unrolled loop to execute the 'left over' iterations. Other strategies +// include generate a loop before or after the unrolled loop. +// +//===----------------------------------------------------------------------===// + +#define DEBUG_TYPE "loop-unroll" +#include "llvm/Transforms/Utils/UnrollLoop.h" +#include "llvm/BasicBlock.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/LoopIterator.h" +#include "llvm/Analysis/LoopPass.h" +#include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/Analysis/ScalarEvolutionExpander.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include "llvm/Transforms/Utils/Cloning.h" +#include + +using namespace llvm; + +STATISTIC(NumRuntimeUnrolled, + "Number of loops unrolled with run-time trip counts"); + +/// Connect the unrolling prolog code to the original loop. +/// The unrolling prolog code contains code to execute the +/// 'extra' iterations if the run-time trip count modulo the +/// unroll count is non-zero. +/// +/// This function performs the following: +/// - Create PHI nodes at prolog end block to combine values +/// that exit the prolog code and jump around the prolog. +/// - Add a PHI operand to a PHI node at the loop exit block +/// for values that exit the prolog and go around the loop. +/// - Branch around the original loop if the trip count is less +/// than the unroll factor. +/// +static void ConnectProlog(Loop *L, Value *TripCount, unsigned Count, + BasicBlock *LastPrologBB, BasicBlock *PrologEnd, + BasicBlock *OrigPH, BasicBlock *NewPH, + ValueToValueMapTy &LVMap, Pass *P) { + BasicBlock *Latch = L->getLoopLatch(); + assert(Latch != 0 && "Loop must have a latch"); + + // Create a PHI node for each outgoing value from the original loop + // (which means it is an outgoing value from the prolog code too). + // The new PHI node is inserted in the prolog end basic block. + // The new PHI name is added as an operand of a PHI node in either + // the loop header or the loop exit block. + for (succ_iterator SBI = succ_begin(Latch), SBE = succ_end(Latch); + SBI != SBE; ++SBI) { + for (BasicBlock::iterator BBI = (*SBI)->begin(); + PHINode *PN = dyn_cast(BBI); ++BBI) { + + // Add a new PHI node to the prolog end block and add the + // appropriate incoming values. + PHINode *NewPN = PHINode::Create(PN->getType(), 2, PN->getName()+".unr", + PrologEnd->getTerminator()); + // Adding a value to the new PHI node from the original loop preheader. + // This is the value that skips all the prolog code. + if (L->contains(PN)) { + NewPN->addIncoming(PN->getIncomingValueForBlock(NewPH), OrigPH); + } else { + NewPN->addIncoming(Constant::getNullValue(PN->getType()), OrigPH); + } + Value *OrigVal = PN->getIncomingValueForBlock(Latch); + Value *V = OrigVal; + if (Instruction *I = dyn_cast(V)) { + if (L->contains(I)) { + V = LVMap[I]; + } + } + // Adding a value to the new PHI node from the last prolog block + // that was created. + NewPN->addIncoming(V, LastPrologBB); + + // Update the existing PHI node operand with the value from the + // new PHI node. How this is done depends on if the existing + // PHI node is in the original loop block, or the exit block. + if (L->contains(PN)) { + PN->setIncomingValue(PN->getBasicBlockIndex(NewPH), NewPN); + } else { + PN->addIncoming(NewPN, PrologEnd); + } + } + } + + // Create a branch around the orignal loop, which is taken if the + // trip count is less than the unroll factor. + Instruction *InsertPt = PrologEnd->getTerminator(); + Instruction *BrLoopExit = + new ICmpInst(InsertPt, ICmpInst::ICMP_ULT, TripCount, + ConstantInt::get(TripCount->getType(), Count)); + BasicBlock *Exit = L->getUniqueExitBlock(); + assert(Exit != 0 && "Loop must have a single exit block only"); + // Split the exit to maintain loop canonicalization guarantees + SmallVector Preds(pred_begin(Exit), pred_end(Exit)); + if (!Exit->isLandingPad()) { + SplitBlockPredecessors(Exit, Preds.data(), Preds.size(), + ".unr-lcssa", P); + } else { + SmallVector NewBBs; + SplitLandingPadPredecessors(Exit, Preds, ".unr1-lcssa", ".unr2-lcssa", + P, NewBBs); + } + // Add the branch to the exit block (around the unrolled loop) + BranchInst::Create(Exit, NewPH, BrLoopExit, InsertPt); + InsertPt->eraseFromParent(); +} + +/// Create a clone of the blocks in a loop and connect them together. +/// This function doesn't create a clone of the loop structure. +/// +/// There are two value maps that are defined and used. VMap is +/// for the values in the current loop instance. LVMap contains +/// the values from the last loop instance. We need the LVMap values +/// to update the inital values for the current loop instance. +/// +static void CloneLoopBlocks(Loop *L, + bool FirstCopy, + BasicBlock *InsertTop, + BasicBlock *InsertBot, + std::vector &NewBlocks, + LoopBlocksDFS &LoopBlocks, + ValueToValueMapTy &VMap, + ValueToValueMapTy &LVMap, + LoopInfo *LI) { + + BasicBlock *Preheader = L->getLoopPreheader(); + BasicBlock *Header = L->getHeader(); + BasicBlock *Latch = L->getLoopLatch(); + Function *F = Header->getParent(); + LoopBlocksDFS::RPOIterator BlockBegin = LoopBlocks.beginRPO(); + LoopBlocksDFS::RPOIterator BlockEnd = LoopBlocks.endRPO(); + // For each block in the original loop, create a new copy, + // and update the value map with the newly created values. + for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) { + BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, ".unr", F); + NewBlocks.push_back(NewBB); + + if (Loop *ParentLoop = L->getParentLoop()) + ParentLoop->addBasicBlockToLoop(NewBB, LI->getBase()); + + VMap[*BB] = NewBB; + if (Header == *BB) { + // For the first block, add a CFG connection to this newly + // created block + InsertTop->getTerminator()->setSuccessor(0, NewBB); + + // Change the incoming values to the ones defined in the + // previously cloned loop. + for (BasicBlock::iterator I = Header->begin(); isa(I); ++I) { + PHINode *NewPHI = cast(VMap[I]); + if (FirstCopy) { + // We replace the first phi node with the value from the preheader + VMap[I] = NewPHI->getIncomingValueForBlock(Preheader); + NewBB->getInstList().erase(NewPHI); + } else { + // Update VMap with values from the previous block + unsigned idx = NewPHI->getBasicBlockIndex(Latch); + Value *InVal = NewPHI->getIncomingValue(idx); + if (Instruction *I = dyn_cast(InVal)) + if (L->contains(I)) + InVal = LVMap[InVal]; + NewPHI->setIncomingValue(idx, InVal); + NewPHI->setIncomingBlock(idx, InsertTop); + } + } + } + + if (Latch == *BB) { + VMap.erase((*BB)->getTerminator()); + NewBB->getTerminator()->eraseFromParent(); + BranchInst::Create(InsertBot, NewBB); + } + } + // LastValueMap is updated with the values for the current loop + // which are used the next time this function is called. + for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end(); + VI != VE; ++VI) { + LVMap[VI->first] = VI->second; + } +} + +/// Insert code in the prolog code when unrolling a loop with a +/// run-time trip-count. +/// +/// This method assumes that the loop unroll factor is total number +/// of loop bodes in the loop after unrolling. (Some folks refer +/// to the unroll factor as the number of *extra* copies added). +/// We assume also that the loop unroll factor is a power-of-two. So, after +/// unrolling the loop, the number of loop bodies executed is 2, +/// 4, 8, etc. Note - LLVM converts the if-then-sequence to a switch +/// instruction in SimplifyCFG.cpp. Then, the backend decides how code for +/// the switch instruction is generated. +/// +/// extraiters = tripcount % loopfactor +/// if (extraiters == 0) jump Loop: +/// if (extraiters == loopfactor) jump L1 +/// if (extraiters == loopfactor-1) jump L2 +/// ... +/// L1: LoopBody; +/// L2: LoopBody; +/// ... +/// if tripcount < loopfactor jump End +/// Loop: +/// ... +/// End: +/// +bool llvm::UnrollRuntimeLoopProlog(Loop *L, unsigned Count, LoopInfo *LI, + LPPassManager *LPM) { + // for now, only unroll loops that contain a single exit + SmallVector ExitingBlocks; + L->getExitingBlocks(ExitingBlocks); + if (ExitingBlocks.size() > 1) + return false; + + // Make sure the loop is in canonical form, and there is a single + // exit block only. + if (!L->isLoopSimplifyForm() || L->getUniqueExitBlock() == 0) + return false; + + // Use Scalar Evolution to compute the trip count. This allows more + // loops to be unrolled than relying on induction var simplification + ScalarEvolution *SE = LPM->getAnalysisIfAvailable(); + if (SE == 0) + return false; + + // Only unroll loops with a computable trip count and the trip count needs + // to be an int value (allowing a pointer type is a TODO item) + const SCEV *BECount = SE->getBackedgeTakenCount(L); + if (isa(BECount) || !BECount->getType()->isIntegerTy()) + return false; + + // Add 1 since the backedge count doesn't include the first loop iteration + const SCEV *TripCountSC = + SE->getAddExpr(BECount, SE->getConstant(BECount->getType(), 1)); + if (isa(TripCountSC)) + return false; + + // We only handle cases when the unroll factor is a power of 2. + // Count is the loop unroll factor, the number of extra copies added + 1. + if ((Count & (Count-1)) != 0) + return false; + + // If this loop is nested, then the loop unroller changes the code in + // parent loop, so the Scalar Evolution pass needs to be run again + if (Loop *ParentLoop = L->getParentLoop()) + SE->forgetLoop(ParentLoop); + + BasicBlock *PH = L->getLoopPreheader(); + BasicBlock *Header = L->getHeader(); + BasicBlock *Latch = L->getLoopLatch(); + // It helps to splits the original preheader twice, one for the end of the + // prolog code and one for a new loop preheader + BasicBlock *PEnd = SplitEdge(PH, Header, LPM->getAsPass()); + BasicBlock *NewPH = SplitBlock(PEnd, PEnd->getTerminator(), LPM->getAsPass()); + BranchInst *PreHeaderBR = cast(PH->getTerminator()); + + // Compute the number of extra iterations required, which is: + // extra iterations = run-time trip count % (loop unroll factor + 1) + SCEVExpander Expander(*SE, "loop-unroll"); + Value *TripCount = Expander.expandCodeFor(TripCountSC, TripCountSC->getType(), + PreHeaderBR); + Type *CountTy = TripCount->getType(); + BinaryOperator *ModVal = + BinaryOperator::CreateURem(TripCount, + ConstantInt::get(CountTy, Count), + "xtraiter"); + ModVal->insertBefore(PreHeaderBR); + + // Check if for no extra iterations, then jump to unrolled loop + Value *BranchVal = new ICmpInst(PreHeaderBR, + ICmpInst::ICMP_NE, ModVal, + ConstantInt::get(CountTy, 0), "lcmp"); + // Branch to either the extra iterations or the unrolled loop + // We will fix up the true branch label when adding loop body copies + BranchInst::Create(PEnd, PEnd, BranchVal, PreHeaderBR); + assert(PreHeaderBR->isUnconditional() && + PreHeaderBR->getSuccessor(0) == PEnd && + "CFG edges in Preheader are not correct"); + PreHeaderBR->eraseFromParent(); + + ValueToValueMapTy LVMap; + Function *F = Header->getParent(); + // These variables are used to update the CFG links in each iteration + BasicBlock *CompareBB = 0; + BasicBlock *LastLoopBB = PH; + // Get an ordered list of blocks in the loop to help with the ordering of the + // cloned blocks in the prolog code + LoopBlocksDFS LoopBlocks(L); + LoopBlocks.perform(LI); + + // + // For each extra loop iteration, create a copy of the loop's basic blocks + // and generate a condition that branches to the copy depending on the + // number of 'left over' iterations. + // + for (unsigned leftOverIters = Count-1; leftOverIters > 0; --leftOverIters) { + std::vector NewBlocks; + ValueToValueMapTy VMap; + + // Clone all the basic blocks in the loop, but we don't clone the loop + // This function adds the appropriate CFG connections. + CloneLoopBlocks(L, (leftOverIters == Count-1), LastLoopBB, PEnd, NewBlocks, + LoopBlocks, VMap, LVMap, LI); + LastLoopBB = cast(VMap[Latch]); + + // Insert the cloned blocks into function just before the original loop + F->getBasicBlockList().splice(PEnd, F->getBasicBlockList(), + NewBlocks[0], F->end()); + + // Generate the code for the comparison which determines if the loop + // prolog code needs to be executed. + if (leftOverIters == Count-1) { + // There is no compare block for the fall-thru case when for the last + // left over iteration + CompareBB = NewBlocks[0]; + } else { + // Create a new block for the comparison + BasicBlock *NewBB = BasicBlock::Create(CompareBB->getContext(), "unr.cmp", + F, CompareBB); + if (Loop *ParentLoop = L->getParentLoop()) { + // Add the new block to the parent loop, if needed + ParentLoop->addBasicBlockToLoop(NewBB, LI->getBase()); + } + + // The comparison w/ the extra iteration value and branch + Value *BranchVal = new ICmpInst(*NewBB, ICmpInst::ICMP_EQ, ModVal, + ConstantInt::get(CountTy, leftOverIters), + "un.tmp"); + // Branch to either the extra iterations or the unrolled loop + BranchInst::Create(NewBlocks[0], CompareBB, + BranchVal, NewBB); + CompareBB = NewBB; + PH->getTerminator()->setSuccessor(0, NewBB); + VMap[NewPH] = CompareBB; + } + + // Rewrite the cloned instruction operands to use the values + // created when the clone is created. + for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) { + for (BasicBlock::iterator I = NewBlocks[i]->begin(), + E = NewBlocks[i]->end(); I != E; ++I) { + RemapInstruction(I, VMap, + RF_NoModuleLevelChanges|RF_IgnoreMissingEntries); + } + } + } + + // Connect the prolog code to the original loop and update the + // PHI functions. + ConnectProlog(L, TripCount, Count, LastLoopBB, PEnd, PH, NewPH, LVMap, + LPM->getAsPass()); + NumRuntimeUnrolled++; + return true; +} -- cgit v1.2.3